The present disclosure relates to a methodology for mitigating mask defects by pattern shifting and a system for implementing the same.
Currently, the mask blanks used in Extreme Ultraviolet (EUV) Lithography cannot be fabricated free of defects. A rapid method of determining the optimum placement of mask patterns on the blank to avoid these defects is described. Using this method, the probability of fabricating defect free masks when the pattern is 1) randomly placed on the mask blank, or 2) positioned optimally to avoid defects, is determined for a variety of integrated circuit designs, defect densities, and defect sizes. In addition to circular defects, oval and clusters of defects are also considered. Finally, simple analytical expressions for the probability of obtaining a defect free mask in the case of random placement of the mask pattern is presented and compared to Monte Carlo simulations.
Photomasks used in conventional optical lithography consist of a fused silica substrate with a patterned optical absorber on the surface. Open regions in the absorber transmit light, typically at 193 nm wavelength, which is imaged onto a silicon wafer. This simple structure coupled with many decades of manufacturing experience has resulted in mask blanks (fused silica with unpatterned absorber) that are substantially free of defects. However, the photomask used in Extreme Ultraviolet Lithography is much more complex, consisting of a fused silica substrate, a distributed Bragg reflector containing ˜50 alternating layers of silicon and molybdenum, a thin capping layer, and finally a patterned absorber layer. In this case, open regions in the absorber reflect the EUV light which is then imaged onto the silicon wafer.
Due to the very short wavelength of 13.6 nm employed in EUV lithography, nanometer scale differences in optical path length produce printable phase defects. Hence, minor pits or bumps in the fused silica substrate of only a few nanometers in height can propagate thru the multilayer mirror and result in printable phase defects. In addition, nanometer scale particles or growth defects embedded in the multilayer minor can also produce printable defects. Thus it is inherently more difficult to fabricate a defect free mask blank for EUV lithography than conventional optical lithography. As a result, EUV mask blanks currently contain numerous defects, a situation which is likely to continue for the foreseeable future.
While several methods for repairing EUV blank defects have been described, a reliable method of repairing every EUV blank defect is not available.